1. Technical Field
The present application relates to an optical microphone for detecting an acoustic signal using light.
2. Description of the Related Art
Microphones for detecting an acoustic signal using a diaphragm and converting the acoustic signal to an electrical signal have been widely used in the art. Such a microphone includes a diaphragm, which is a mechanically vibrating portion. Therefore, the characteristics of the diaphragm may possibly deteriorate while using the microphone repeatedly. When an acoustic signal of a high intensity is detected using the microphone, the diaphragm of the microphone may possibly be vibrated significantly by the acoustic signal, thereby breaking the diaphragm.
Moreover, one cannot expect a microphone to detect a desirable acoustic signal at frequencies greater than or equal to the lowest resonance frequency of the diaphragm. Therefore, it is difficult for a microphone to detect frequencies over a wide range (hereinafter referred to as the “wide-range property”.). When the diaphragm is downsized in order to obtain a wide-range property, the acoustic signal detection sensitivity decreases.
In contrast, Japanese Laid-Open Patent Publication No. 2009-85868 (hereinafter, referred to as “Patent Document No. 1”), for example, proposes an optical microphone for detecting an acoustic signal using light and converting the acoustic signal to an electric signal. A conventional optical microphone disclosed in Patent Document No. 1 will now be described.
FIG. 20 shows a conventional optical microphone 141 disclosed in Patent Document No. 1. The optical microphone 141 includes a receiving mechanism section 1410 and a laser Doppler vibrometer 148.
The receiving mechanism section 1410 includes a base portion 143 having a depressed portion, and a transparent support plate 145 transparent to a laser beam 147. The depressed portion of the base portion 143 and the transparent support plate 145 together form a space. This space is filled with a nanoporous material 142 which is a propagation medium.
A medium whose sound velocity is slow (small acoustic impedance) and which has a small acoustic propagation loss, such as a dry silica gel disclosed in Japanese Patent No. 3633926 (hereinafter, referred to as “Patent Document No. 2”), for example, is used as the nanoporous material 142. The base portion 143 includes an opening 144 for introducing an acoustic signal 149 to the nanoporous material 142. The bottom surface of the depressed portion of the base portion 143 is a reflective surface 1411.
The laser Doppler vibrometer 148 outputs the laser beam 147 from outside the receiving mechanism section 1410. The laser beam 147 passes through the transparent support plate 145 and the nanoporous material 142 to be reflected at the reflective surface 1411. The laser beam 147, which has been reflected at the reflective surface 1411, passes through the nanoporous material 142 and the transparent support plate 145 to return to the laser Doppler vibrometer 148.
The ambient acoustic signal 149 enters the receiving mechanism section 1410 through the opening 144 and is refracted at the interface between the air and the nanoporous material 142 to enter the nanoporous material 142 with a high efficiency. The incident acoustic signal 149 is converted to a compressional wave 1412 which travels through the nanoporous material 142. At the spot position, on the nanoporous material 142, of the laser beam 147 output from the laser Doppler vibrometer 148, the produced compressional wave 1412 is observed as a temporal fluctuation in density. Since the density change causes a refractive index change, a temporal fluctuation in refractive index occurs at the spot position in accordance with the acoustic signal 149.
As shown in FIG. 20, when the laser beam 147 is allowed to enter from the direction normal to the interface between the nanoporous material 142 and the transparent support plate 145, the amount of fluctuation in phase which the laser beam 147 having passed therethrough undergoes due to the temporal fluctuation in refractive index is optically equivalent to the amount of fluctuation in phase to be undergone on the assumption that the reflective surface 1411 were in oscillatory motion in the normal direction in accordance with the temporal fluctuation in refractive index at the spot position. Therefore, the laser beam 147 reflected and returning from the reflective surface 1411 undergoes a Doppler shift that associated with the oscillatory motion of the reflective surface 1411. The laser Doppler vibrometer 148 measures the light component which has undergone the Doppler shift included in the returning laser beam 147 having been reflected at the reflective surface 1411. The light intensity for each amount of frequency shift is detected by obtaining the Fourier coefficient with respect to the amount of frequency shift of the light component.
The fluctuation in refractive index is generally in proportion to the sound pressure of the acoustic signal 149, and the amount of Doppler shift (amount of frequency change) is in proportion to the velocity of the oscillatory motion. Therefore, the laser Doppler vibrometer 148 outputs a signal that is generally in proportion to the time derivative of the acoustic signal 149. By performing an integration operation or an appropriate filter operation on the output signal, it is possible to obtain an electric signal associated with the acoustic signal. Thus, the optical microphone 141 can be made to operate as a microphone having intended acoustic characteristics.